I cannot search configurable products in Fast Order Form. Could you please suggest solution?

Answer:

This is seemingly because configurable product does not have sub-products.

In Magento 2, if a configurable product does not have children products, it will be automatically changed into simple product.

However in your case, if you import mass of products to Fast Order, it's not auto-change into children product. Hence, the search is error. Please check check whether configurable product has sub-products, if not, you need to disable that configurable product.